1. Field of the Invention
The invention relates to power transmission gearing including a countershaft with a ratio change clutch for establishing high ratio and low ratio torque delivery paths through countershaft gear elements.
2. Background Art
A power transmission mechanism for road vehicles, both automotive passenger vehicles and heavy-duty trucks and medium-duty trucks, have multiple gear ratios in a range that is sufficiently broad to accommodate torque capacity requirements. An increased overall gear ratio range may be provided by using, with a multiple-ratio transmission mechanism, a range gearing arrangement. The overall number of gear ratios that would be available using range gearing would be equal to the number of gear ratios for the transmission multiplied by the number the number of ratios available in the range gearing arrangement. An example of a dual clutch transmission with range gearing is disclosed in pending application Ser. No. 11/702,361, filed Feb. 5, 2007, entitled “Dual Clutch Transmission with Multiple Range Gearing”. That application is assigned to the assignee of the present invention.
Ratio changes in a multiple-ratio transmission may be achieved by selectively engaging and disengaging dual clutch discs in a master clutch assembly that drivably connects a vehicle engine to power input headset gearing for the multiple-ratio transmission. An example of a dual clutch construction capable of being used in transmissions of this type may be seen by referring to co-pending U.S. patent application Ser. No. 10/983,531, filed Nov. 8, 2004, entitled “Dual Clutch Assembly for a Heavy-Duty Automotive Powertrain”, now U.S. Pat. No. 7,204,166, issued Apr. 17, 2007. That application also is assigned to the assignee of the present invention. An earlier example of a dual clutch arrangement capable of being used in a countershaft transmission is disclosed in U.S. Pat. No. 6,397,994.
In these examples of dual clutch constructions, separate master clutch discs deliver power to each of two power input shafts, one of which may be concentrically disposed within the other. The clutch discs may be selectively engaged and disengaged hydraulically or mechanically so that as one clutch disc becomes disengaged, the other becomes engaged, and vice-versa. A vehicle engine thus can be selectively connected to each of the two power input shafts during a power shift sequence.
It is known design practice also to increase the number of overall powertrain ratios by providing a gearing arrangement with additional headset gearing rather than a range gearing arrangement that is separate from the transmission gearing. The additional headset gearing, however, together with an added master friction clutch disc that would be required for the additional headset gear, would result in added cost, added weight and complexity to the powertrain.